The present application is generally related to optical communication and signal generation devices, systems, and methods. In one embodiment, the invention provides a multiple channel fiberoptic light source, particularly for use in dense wavelength division multiplexed systems (DWDM).
Optical telecommunications networks would benefit from increased bandwidth to handle both the current and projected communications traffic. Many optical networks use a single laser transmitter with time division multiplexing techniques to transmit separate data streams. The advantageous properties of optical fiber allow quite significant data rates to be transferred using such single laser systems. Nonetheless, to allow even greater data rates to be transmitted, the current market trend is toward systems that use many different wavelengths. Simultaneous transmission of these different wavelength signals can significantly increase data capacity of a fiberoptic communication link. This approach is known generally as wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d).
WDM systems have significantly increased the capacity of each fiberoptic communication link within the system. As with many successes, still further improvements would be desirable. To increase the number of channels or distinct frequencies transmitted across an optical fiber, it has been proposed to decrease the frequency separation between adjacent channels so as to more densely transmit data within a spectral range. Such dense wavelength division multiplexing (xe2x80x9cDWDMxe2x80x9d) optical communications networks show great promise in providing increased data communication capabilities and bandwidth. Unfortunately, as the adjacent channels get closer and closer in spacing, the likelihood of error (from signal frequency instability, cross-talk, and the like) increases.
The reliability and accuracy of data transmissions using DWDM systems are particularly sensitive to the stability and reliability of the light generating structures. A variety of techniques have been used for individually controlling each laser which generates the light for a particular channel within a DWDM system. While the cost of individual laser diodes has dramatically decreased, the price of a multiple wavelength (channel) light generation devices, together with control systems for accurately maintaining stable and tightly packed channel frequencies, can be quite expensive.
In work related to the present invention, it has recently been proposed to flexibly combine a plurality of tunable laser diodes together so as to form a reconfigurable multi-channel transmitter for DWDM optical communications. U.S. patent application Ser. No. 09/610,312 filed on Jul. 5, 2000 (assigned to the assignee of the present invention) describes this advantageously reconfigurable multi-channel transmitter in detail, and the full disclosure of that application is incorporated herein by reference.
While the recently proposed reconfigurable multi-channel transmitter represents a significant advancement in the art, still further refinements would be desirable. In general, it would be beneficial to provide improved structures, systems, and methods for generating light for use in optical signal systems. In particular, it would be beneficial to provide improved light sources for fiberoptic communications. It would be particularly beneficial to provide improved combinations and/or arrangements of components for generating optical signaling light, ideally resulting in simple structures and providing highly reliable and stable light at a plurality of tightly controlled channel frequencies for a reasonable cost.
The present invention generally provides improved structures and systems for generating light for use in optical communications systems, together with related methods for their fabrication and/or use. In one embodiment, the invention provides an optical light source for use in wavelength division multiplexed and dense wavelength division multiplexed systems. In some embodiments, a reference gas may be contained by a hermetically sealed enclosure of the device itself, which is also used to protect lasers or other light generating structures. Laser injection current control and thermal management systems may together provide feedback control of the light generating lasers and/or a frequency grid generator such as an etalon. Electronic control over a number of separate lasers (and the grid) may be effected using a multiplexed digital controller.
In a first aspect, the invention provides a light generating structure comprising a sealed enclosure with a light source disposed within the enclosure. A waveguide has an entrance disposed within the enclosure in an optical path from the light source. A reference fluid is contained by the enclosure itself. The reference fluid imposes a reference characteristic on light from the light source.
The waveguide may comprise an optical fiber, for example, which can transport light outside the enclosure. In many embodiments, the reference characteristics will comprise a reference wavelength. The reference fluid will often comprise a reference gas, and may absorb light energy at the reference wavelength. The light source will often comprise a frequency adjustable laser, and a sensor may couple the light (with the reference characteristic imposed thereon) to the frequency adjustable laser along a feedback control circuit so as to frequency stabilize the laser by use of the reference wavelength.
Advantageously, a plurality of lasers can be disposed within the enclosure, with the reference gas imposing the reference characteristic upon light traveling from each laser. The waveguide will often comprise an optical fiber, the waveguide entrance comprising a first end of the optical fiber, and the optical fiber extending outside the enclosure to a second optical fiber end. Additional optical fibers may similarly be provided for the additional lasers for transmission of light from the lasers out of the enclosure.
The use of (for example) a hermetically sealed enclosure which protects the lasers as a reference gas cell significantly facilitates the miniaturization of light sources having reference gas stabilized frequencies. Fiberoptic enclosures are often quite small, and accurate detection of the reference wavelength may be easier if the light travels a significant distance through the reference gas. Hence, a length of the optical path through the reference fluid within the enclosure will often be greater than a length of the enclosure, the optical path typically comprising a folded optical path reflected by at least one reflecting element within the enclosure. This allows, for example, the reference gas to sufficiently absorb energy along a characteristic wavelength for easy and accurate wavelength control, without having to resort to a large, separately sealed gas cell disposed within the enclosure.
In another aspect, the invention provides a fiberoptic source comprising a sealed enclosure, with a first variable-frequency laser disposed within the enclosure. An optical fiber has an entrance disposed within the enclosure in an optical path from the laser. The fiber extends out from the enclosure, and a reference gas is contained by the enclosure. The reference gas tags light from the laser with a reference frequency. Feedback control circuitry couples the tagged light to the laser so as to control a frequency of the light in response to the reference frequency.
The reference gas typically tags the light via light absorption, which occurs at a characteristic frequency or frequencies of the (often narrow) absorption lines of the gas. In many embodiments, a plurality of frequency-adjustable lasers will be provided, each laser producing an additional light signal having a signal frequency. The signal frequencies may be adjusted in response to the reference frequency. A frequency adjustable etalon may be disposed in an optical path from at least one of the lasers. The etalon may generate interference fringes at a plurality of discretely separated frequencies. An etalon control loop may be coupled to the etalon to stabilize the separated frequencies of the etalon in response to the reference frequency. The feedback control circuitry may include a plurality of control loops coupled to the additional lasers for adjustment of the signal frequencies of the additional lasers in response to the separated frequencies of the etalon. The feedback control circuitry may comprise a multiplexer disposed along the feedback control loops so as to provide sequential adjustment of the etalon and the lasers. In some embodiments, the optical path from the first laser to the entrance of the optical fiber will be disposed adjacent a first end of a laser cavity, while the light to be tagged by the reference gas may be refracted from the second end of the laser cavity.
In another aspect, the invention provides a light generation structure comprising a plurality of variable-frequency light sources. The light sources each generate an associated light signal. A frequency grid generator has a structure defining a plurality of discrete fringe frequencies. A spacing between the discrete frequencies is substantially fixed. A digital controller transmits signals to selectively alter frequencies of the generated light. A multiplexer sequentially couples the controller to the light sources and the frequency grid generator so as to maintain each of the light signals in alignment with an associated discrete frequency of the frequency grid.
Typically, the frequency grid generator will comprise a variable frequency grid generator having optical interference fringes defining a frequency comb, and allowing the discrete frequencies of the comb to be varied in unison, the exemplary grid generator comprising an optical etalon or the like. A frequency reference may provide frequency reference information to the digital controller, so as to effect control over the position of discrete frequencies of the frequency comb. Control over the frequency grid may be effected by controlling temperature of the grid generator, ideally using a resistive heater and a thermistor for coarse temperature feedback.
In some embodiments, light sensors may be coupled to the digital controller for sensing associated light frequencies of light from each of the lasers, while temperature sensors are coupled to each of the lasers for sensing an associated temperature thereof. Heaters may also be thermally coupled to each laser, with the digital controller effecting temperature feedback control of the lasers. Frequency feedback control of the lasers may be effected in response to the associated light frequency as sensed by the light sensors, while the temperature feedback control may maintain a frequency adjustment range of each laser along a desired range. This allows, for example, high speed frequency tuning of the laser light to be effected by adjusting the injection current (for maintaining a signal at a desired channel frequency, for example) while using thermal control of the laser for larger, slower changes (for example, to select the channel for a particular laser, to maintain the injection current roughly centered within an adjustment range, and the like).
In another aspect, the invention provides a fiberoptic source comprising a plurality of variable-frequency lasers. Each laser generates light having a frequency which varies with the temperature of the laser. A frequency grid generator has a structure defining a plurality of discrete interference fringe frequencies. The discrete frequencies vary (often as a set) with the temperature of the grid generator. A controller is coupled to the lasers and the frequency grid generator. The controller varies the temperature of the grid generator in response to a reference frequency by selectively heating the grid generator. The controller varies the temperatures of each lasers to associate the lasers with discrete frequencies of the grid generator by selectively heating the lasers. A heat sink is thermally coupled to the lasers and the grid generator. One of the lasers or the grid generator defines a minimum desired cooling device, typically establishing a minimum desired cooling level. The heat sink maintains the minimum desired cooling device sufficiently cool to allow the controller, via the heaters, to effect feedback control of the light frequencies, optionally to associate the frequencies of the laser light with desired discrete frequencies of the grid generator.
Typically, each laser generates light having a frequency which varies with injection current, as well as with the temperature of the laser. An etalon is often employed as the grid generator, the etalon defining a frequency comb. The discrete fringe frequencies of the etalon vary relatively slowly (often as a set) with the temperature of the etalon. By varying the etalon temperature in response to a gas absorption line or other reference frequency, the frequency grid or comb generated by the etalon can have a fringe positioned so that it coincides with the reference frequency. This can be achieved by tuning the etalon to the position of transparency at the frequency of a laser tuned to the absorption line. In this way, the frequency grid of the etalon becomes referenced or xe2x80x9canchoredxe2x80x9d to an absolute reference. Typically, a thermoelectric cooler (TEC) maintains the temperature of the structure at the lowest point required by any of the devices on the source. All other devices can be brought to their respective operating temperatures by selective localized heating performed by associated microheaters attached to laser submounts or the etalon plate.
In yet another aspect, the invention provides a fiberoptic source assembly comprising an enclosure with a plurality of lasers disposed within the enclosure. A plurality of waveguide entrances are provided, with each entrance in a first optical path from an associated laser. A plurality of sensors are optically coupled to the lasers, and a controller couples the sensors to the lasers so as to effect independent feedback control over the light generated by the lasers.
In some embodiments, each laser will have a cavity with a first end and a second end, and the waveguide entrances will be optically coupled to the laser cavity through the first end. The sensors may be disposed in a second optical path from the second end of the lasers, thereby taking advantage of what is sometimes considered xe2x80x9cleakagexe2x80x9d light emanating from the back end of the laser cavity. The lasers may be disposed between first and second sets of reflecting elements so that at least one of the optical paths within the enclosure is longer than the enclosure. Each reflecting element may comprise a prism, mirror, or the like, and may be used for a plurality of separated optical paths, for example, by folding the optical paths along parallel planes and positioning the lasers so that the optical path planes are separated along the reflecting prisms. In some embodiments, a plane or substrate may support the sensors. Conveniently, the substrate may extend across the light generated by the lasers, with a portion of a folded optical path from the lasers optionally traversing through the substrate at a window or opening through the substrate.